Keep my 6.5m at Eastlands Boatyard which is up river under the motorway bridge passed Swanick and Bursledon. They have a 'trailor sailor' facility as the slipway is tidal and usable approx 2 hours each side of high tide. You can either moor up on a holding pontoon until you want to get the boat out or they offer launch and recovery service when the tide isnt right for launching which is £25 in and out or £15 one way. All you have to do is call the day before so when you get there the boat is in the water and ready to go. I am paying £91 plus vat per month plus launch and recovery fees as appropriate. You have to commit to 3 months at a time.
Seems pretty secure, although there was a story recently of someone nicking an outboard from there which did require them to carry the thing acroos the fields which shows they were pretty determined! There is a heafty pin number controlled gate for out of hours and also good clean toilets and shower on site which are also pin number controlled. Ample parking also.
Also its on the same site as Rib Shop for repairs and servicing if required which can save alot of messing about delivering boats and so on when your outboard needs looking at or general repairs.
They do offer mooring too so suggest you visit www.eastlandsboatyard.com
I guess the down side is the distance it is up river - so a good 40 min chug to the solent which can be a bit of a drag.
Hope this helps!